Thick film conductive composition and process for use in the manufacture of semiconductor device

ABSTRACT

The present invention is directed to a thick film conductive composition comprising: a) electrically conductive silver powder; b) ZnO powder; c) lead-free glass frits wherein based on total glass frits: Bi 2 O 3 : &gt;5 mol %, B 2 O 3 : &lt;15 mol %, BaO: &lt;5 mol %, SrO: &lt;5 mol %, Al 2 O 3 : &lt;5 mol %; and d) organic medium, wherein (the content of ZnO/the content of the silver powder)×100 is more than 2.5.

FIELD OF THE INVENTION

This invention is directed primarily to a silicon semiconductor device.In particular, it is directed to a conductive silver paste for use inthe front side of a solar cell device.

TECHNICAL BACKGROUND OF THE INVENTION

The present invention can be applied to a broad range of semiconductordevices, although it is especially effective in light-receiving elementssuch as photodiodes and solar cells. The background of the invention isdescribed below with reference to solar cells as a specific example ofthe prior art.

A conventional solar cell structure with a p-type base has a negativeelectrode that is typically on the front-side or sun side of the celland a positive electrode on the backside. It is well-known thatradiation of an appropriate wavelength falling on a p-n junction of asemiconductor body serves as a source of external energy to generatehole-electron pairs in that body. Because of the potential differencewhich exists at a p-n junction, holes and electrons move across thejunction in opposite directions and thereby give rise to flow of anelectric current that is capable of delivering power to an externalcircuit. Most solar cells are in the form of a silicon wafer that hasbeen metallized, i.e., provided with metal contacts that areelectrically conductive.

Most terrestrial electric power-generating solar cells currently aresilicon solar cells. Process flow in mass production is generally aimedat achieving maximum simplification and minimizing manufacturing costs.Electrodes in particular are made by using a method such as screenprinting to form a metal paste. An example of this method of productionis described below in conjunction with FIG. 1.

FIG. 1A shows a p-type silicon substrate, 10.

In FIG. 1B, an n-type diffusion layer, 20, of the reverse conductivitytype is formed by the thermal diffusion of phosphorus (P) or the like.Phosphorus oxychloride (POCl₃) is commonly used as the phosphorusdiffusion source. In the absence of any particular modification, thediffusion layer, 20, is formed over the entire surface of the siliconsubstrate, 10. This diffusion layer typically has a sheet resistivity onthe order of several tens of ohms per square (Ω/□), and a thickness ofabout 0.3 to 0.5 μm.

After protecting one surface of this diffusion layer with a resist orthe like, as shown in FIG. 1C, the diffusion layer, 20, is removed frommost surfaces by etching so that it remains only on one main surface.The resist is then removed using an organic solvent or the like.

Next, a silicon nitride film, 30, is formed as an anti-reflectioncoating on the n-type diffusion layer, 20, to a thickness of typicallyabout 700 to 900 Å in the manner shown in FIG. 1D by a process such asplasma chemical vapor deposition (CVD).

As shown in FIG. 1E, a silver paste, 500, for the front electrode isscreen printed then dried over the silicon nitride film, 30. Inaddition, a backside silver or silver/aluminum paste, 70, and analuminum paste, 60, are then screen printed and successively dried onthe backside of the substrate. Firing is then carried out in an infraredfurnace at a temperature range of approximately 700° C. to 975° C. for aperiod of from several minutes to several tens of minutes.

Consequently, as shown in FIG. 1F, aluminum diffuses from the aluminumpaste into the silicon substrate, 10, as a dopant during firing, forminga p+ layer, 40, containing a high concentration of aluminum dopant. Thislayer is generally called the back surface field (BSF) layer, and helpsto improve the energy conversion efficiency of the solar cell.

The aluminum paste is transformed by firing from a dried state, 60, toan aluminum back electrode, 61. The backside silver or silver/aluminumpaste, 70, is fired at the same time, becoming a silver orsilver/aluminum back electrode, 71. During firing, the boundary betweenthe back side aluminum and the back side silver or silver/aluminumassumes an alloy state, and is connected electrically as well. Thealuminum electrode accounts for most areas of the back electrode, owingin part to the need to form a p+ layer, 40. Because soldering to analuminum electrode is impossible, a silver back electrode is formed overportions of the back side as an electrode for interconnecting solarcells by means of copper ribbon or the like. In addition, the frontelectrode-forming silver paste, 500, sinters and penetrates through thesilicon nitride film, 30, during firing, and is thereby able toelectrically contact the n-type layer, 20. This type of process isgenerally called “fire through.” This fired through state is apparent inlayer 501 of FIG. 1F.

JP-2001-313400A to Fujii et al. teaches a solar cell which is obtainedby forming, on one main surface of a semiconductor substrate, regionsthat exhibit the other type of conductivity and forming anantireflection coating on this main surface of the semiconductorsubstrate. The resulting solar cell has an electrode material coatedover the antireflection coating and fired. The electrode materialincludes, for example, lead, boron and silicon, and additionallycontains, in a glass frit having a softening point of about 300 to 600°C., and one or more powders from among titanium, bismuth, cobalt, zinc,zirconium, iron, and chromium.

U.S. Pat. No. 4,737,197 to Nagahara et al. discloses a solar cellincluding a semiconductor substrate, a diffused layer provided in thesemiconductor substrate by diffusion of dopant impurities, and a contactmade of metal paste formed on the diffusion layer. The metal pasteincludes metal powder, which functions as the main contact material,glass frits, an organic binder, a solvent, and an element belonging tothe fifth group of the periodic table.

U.S. Patent Publication 2006-0231801 A1 to Alan et al. teaches a thickfilm conductive composition comprising: (a) electrically conductivesilver powder; (b) zinc-containing additive; (c) glass frit wherein saidglass frit is lead-free; dispersed in (d) organic medium. As a preferredglass, the following composition is referred to: SiO₂ 0.1-8 wt %, Al₂O₃0-4 wt %, B₂O₃ 8-25 wt %, CaO 0-1 wt %, ZnO 0-42 wt %, Na₂O 0-4 wt %,Li₂O 0-3.5 wt %, Bi₂O₃ 28-85 wt %, Ag₂O 0-3 wt %, CeO₂ 0-4.5 wt %, SnO₂0-3.5 wt %, and BiF₃ 0-15 wt %.

U.S. Patent Publication 2006-0289055 to Sridharan et al. teaches a solarcell comprising a contact made from a mixture wherein, prior to firing,the mixture comprises: solids portion and organics portion, wherein thesolids portion comprises (i) from about 85 to about 99 wt % of aconductive metal component, and (ii) from about 1 to about 15 wt % of aglass component, wherein the glass component is lead-free.

Although, as noted, various methods and compositions for forming solarcells exist, there is an on-going effort to provide compositions whichare Pb-free while at the same time maintaining electrical performance.The present inventors create a novel composition and method ofmanufacturing a semiconductor device which provides such a Pb-freesystem and maintains electrical performance and solder adhesion.

SUMMARY OF THE INVENTION

The present invention is directed to a thick film conductive compositioncomprising: a) electrically conductive silver powder; b) ZnO powder; c)lead-free glass frits wherein based on total glass frits: Bi₂O₃: >5 mol%, B₂O₃: <15 mol %, BaO: <5 mol %, SrO: <5 mol %, Al₂O₃: <5 mol %; andd) organic medium, wherein (the content of ZnO/the content of the silverpowder)×100 is more than 2.5.

The present invention is further directed to an electrode formed fromthe composition above wherein said composition has been fired to removethe organic medium and to sinter said glass particles. Still further,the invention is directed to a method of manufacturing a semiconductordevice from a structural element composed of a semiconductor having ap-n junction and an insulating film formed on a main surface of thesemiconductor comprising the steps of (a) applying onto said insulatingfilm the thick film composition detailed above; and (b) firing saidsemiconductor, insulating film and thick film composition to form anelectrode. Additionally, the present invention is directed to asemiconductor device formed by the method detailed above and asemiconductor device formed from the thick film conductive compositiondetailed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram illustrating the fabrication of asemiconductor device.

FIG. 2 is a graph showing the relationship between Bi₂O₃ and Rc.

FIG. 3 is a graph showing the relationship between BaO and Rc.

FIG. 4 is a graph showing the relationship between SrO and Rc.

FIG. 5 is a graph showing the relationship between B₂O₃ and Rc.

FIG. 6 is a graph showing the relationship between Al₂O₃ and Rc.

FIG. 7 is a graph showing the relationship between (ZnO/Ag)×100 and Eff.

DETAILED DESCRIPTION OF THE INVENTION

The main components of the thick film conductor composition(s) areelectrically functional silver powders, ZnO powder, and Pb-free glassfrit dispersed in an organic medium. The glass frit of the presentinvention has prescribed content of Bi₂O₃, B₂O₃, BaO, SrO, and Al₂O₃.Further, the composition of the present invention meets a specificrelationship between ZnO and Ag. Additional additives may includemetals, metal oxides or any compounds that can generate these metaloxides during firing. The components are discussed herein below.

Inorganic Components

The inorganic components of the present invention comprise (1)electrically functional silver powders; (2) zinc oxide (ZnO); and (3)Pb-free glass frit; and optionally (4) additional metal/metal oxideadditive selected from (a) a metal selected from Zn, Gd, Ce, Zr, Ti, Mn,Sn, Ru, Co, Fe, Cu and Cr; (b) an oxide of a metal selected from Zn, Gd,Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu and Cr; (c) any compounds that cangenerate the metal oxides of (b) upon firing; and (d) mixtures thereof.

A. Electrically Functional Silver Powders

Generally, a thick film composition comprises a functional phase thatimparts appropriate electrically functional properties to thecomposition. The functional phase comprises electrically functionalpowders dispersed in an organic medium that acts as a carrier for thefunctional phase that forms the composition. The composition is fired toburn out the organic phase, activate the inorganic binder phase and toimpart the electrically functional properties.

The functional phase of the composition may be coated or uncoated silverparticles which are electrically conductive. When the silver particlesare coated, they may be partially coated with a surfactant. Thesurfactant may be selected from, but is not limited to, stearic acid,palmitic acid, a salt of stearate, a salt of palmitate and mixturesthereof. Other surfactants may be utilized including lauric acid,palmitic acid, oleic acid, stearic acid, capric acid, myristic acid andlinolic acid. The counter-ion can be, but is not limited to, hydrogen,ammonium, sodium, potassium and mixtures thereof.

The particle size of the silver is not subject to any particularlimitation, although an average particle size of no more than 10 μm, andpreferably no more than 5 μm, is desirable. The silver powder accountsfor, but not limited to, 70 to 85 wt % of the paste composition, andgenerally 85 to 99 wt % of the solids in the composition (i.e.,excluding the organic vehicle).

B. Zinc Oxide (ZnO) Powder

In one embodiment, ZnO powder has an average particle size in the rangeof 10 nm to 10 μm. In a further embodiment, the ZnO powder has anaverage particle size of 40 nm to 5 μm. In still a further embodiment,the ZnO powder has an average particle size of 60 nm to 3 μm.

As shown in the following example section, it was found that higher ZnOin comparison to Ag upgrades electric performance of formed electrode.For example, the efficiency of solar is improved. In this context, thecontent of ZnO and the content of Ag meet the following formula.(the content of ZnO/the content of the silver powder)×100>2.5

The value of the formula is preferably more than 4.0, and morepreferably more than 5.0.

Typically, ZnO is present in the composition in the range of 0.5 to 15.0weight percent total composition. In one embodiment, the ZnO is presentin the range of 2.0 to 7.0 weight percent total composition.

C. Glass Frit

As shown in the following example section, higher Bi₂O₃ content upgradeselectric performance of formed electrode. For example, the contactresistance between solar cell and electrode is reduced, resulting in thesuperior performance of formed solar cell. To the contrary, higher B₂O₃,BaO, SrO, and Al₂O₃ content of the glass degrades performance. In thiscontext, the glass frit of the present invention contains the followingoxide constituents at the prescribed range.

-   -   Bi₂O₃: >5 mol %    -   B₂O₃: <15 mol %    -   BaO: <5 mol %    -   SrO: <5 mol %    -   Al₂O₃: <5 mol %.

Bi₂O₃ is contained preferably in the range of 10.0 to 60.0 mol %. Thecontent of B₂O₃ is preferably less than 15.0 mol %, more preferably lessthan 10.0 mol %. The content of BaO is preferably less than 2.0 mol %,more preferably substantially 0 mol %. The content of SrO is preferablyless than 2.0 mol %, more preferably 0 mol %. The content of Al₂O₃ ispreferably less than 2.0 mol %, more preferably 0 mol %.

As examples of glass frit, glass frit that contains Bi₂O₃ as a maincomponent may be used. In case that Bi₂O₃ is used as a main component ofglass frit, the content of Bi₂O₃ is preferably 10.0-60.0 mol %. Thecontent of B₂O₃ is preferably 0.0-15.0 mol %.

Substitutions of glass formers such as P₂O₅ 0-3, GeO₂ 0-3, V₂O₅ 0-3 inweight % may be used either individually or in combination to achievesimilar performance. It is also possible to substitute one or moreintermediate oxides, such as TiO₂, Ta₂O₅, Nb₂O₅, ZrO₂, CeO₂, SnO₂ forother intermediate oxides (i.e., CeO₂, SnO₂) present in a glasscomposition of this invention. The CaO, alkaline earth content, can alsobe partially or fully replaced by other alkaline earth constituents suchas MgO although CaO maybe preferred.

An average particle size in the range of 1.0-4.0 μm is preferred. Thesoftening point of the glass frit (Ts: second transition point of DTA)is preferred to be in the range of 450-650° C. The amount of glass fritin the total composition is preferably in the range of 0.5 to 7.0 wt. %of the total composition. In one embodiment, the glass composition ispresent in the amount of 0.5 to 4.0 weight percent total composition. Ina further embodiment, the glass composition is present in the range of1.0 to 3.0 weight percent total composition.

The glasses described herein are produced by conventional glass makingtechniques. The glasses can be prepared in 500-1000 gram quantities.Typically, the ingredients are weighed then mixed in the desiredproportions and heated in a bottom-loading furnace to form a melt inplatinum alloy crucibles. Heating is conducted to a peak temperature(1000° C.-1200° C.) and for a time such that the melt becomes entirelyliquid and homogeneous. The molten glass is quenched between counterrotating stainless steel rollers to form a 10-20 mil thick platelet ofglass. The resulting glass platelet is then milled to form a powder withits 50% volume distribution set between 1-3 μm.

D. Additional Metal/Metal Oxide Additives

The additional metal/metal oxide additives of the present invention maybe selected from (a) a metal wherein said metal is selected from Gd, Ce,Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, and Cr; (b) a metal oxide of one or moreof the metals selected from Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu andCr; (c) any compounds that can generate the metal oxides of (b) uponfiring; and (d) mixtures thereof.

The particle size of the additional metal/metal oxide additives is notsubject to any particular limitation, although an average particle sizeof no more than 10 μm, and preferably no more than 5 μm, is desirable.

In one embodiment, the particle size of the metal/metal oxide additiveis in the range of 10 nanometers (nm) to 10 micrometer (μm).

Organic Components

E. Organic Medium

The inorganic components are typically mixed with an organic medium bymechanical mixing to form viscous compositions called “pastes”, havingsuitable consistency and rheology for printing or coating. A widevariety of inert viscous materials can be used as organic medium. Theorganic medium must be one in which the inorganic components aredispersible with an adequate degree of stability. The rheologicalproperties of the medium is preferred to be such that they lend goodapplication properties to the composition, including: stable dispersionof solids, appropriate viscosity and thixotropy for screen printing,appropriate wettability of the substrate and the paste solids, a gooddrying rate, and good firing properties. The organic vehicle used in thethick film composition of the present invention is preferably anonaqueous inert liquid. Use can be made of any of various organicvehicles, which may or may not contain thickeners, stabilizers and/orother common additives. The organic medium is typically a solution ofpolymer(s) in solvent(s). Additionally, a small amount of additives,such as surfactants, may be a part of the organic medium. The mostfrequently used polymer for this purpose is ethyl cellulose. Otherexamples of polymers include ethylhydroxyethyl cellulose, wood rosin,mixtures of ethyl cellulose and phenolic resins, polymethacrylates oflower alcohols, and monobutyl ether of ethylene glycol monoacetate canalso be used. The most widely used solvents found in thick filmcompositions are ester alcohols and terpenes such as alpha- orbeta-terpineol or mixtures thereof with other solvents such as kerosene,dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexyleneglycol and high boiling alcohols and alcohol esters. In addition,volatile liquids for promoting rapid hardening after application on thesubstrate can be included in the vehicle. Various combinations of theseand other solvents are formulated to obtain the viscosity and volatilityrequirements desired.

The polymer present in the organic medium is in the range of 8 wt % to11 wt % of the total composition. The thick film silver composition ofthe present invention may be adjusted to a predetermined,screen-printable viscosity with the organic medium.

The ratio of organic medium in the thick film composition to theinorganic components in the dispersion is dependent on the method ofapplying the paste and the kind of organic medium used, and it can vary.Usually, the dispersion will contain 70-95 wt % of inorganic componentsand 5-30 wt % of organic medium (vehicle) in order to obtain goodwetting.

Description of Method of Manufacturing a Semiconductor Device

Accordingly, the invention provides a novel composition(s) that may beutilized in the manufacture of a semiconductor device. The semiconductordevice may be manufactured by the following method from a structuralelement composed of a junction-bearing semiconductor substrate and asilicon nitride insulating film formed on a main surface thereof. Themethod of manufacture of a semiconductor device includes the steps ofapplying (typically, coating and printing) onto the insulating film, ina predetermined shape and at a predetermined position, the conductivethick film composition of the present invention having the ability topenetrate the insulating film, then firing so that the conductive thickfilm composition melts and passes through the insulating film, effectingelectrical contact with the silicon substrate. The electricallyconductive thick film composition is a thick-film paste composition, asdescribed herein, which is made of a silver powder, ZnO powder, a glassor glass powder mixture having a softening point of 450 to 650° C.,dispersed in an organic vehicle and optionally, additional metal/metaloxide additive(s).

The composition has a glass powder content of less than 7% by weight ofthe total composition and ZnO powder combined with optional additionalmetal/metal oxide additive content of no more than 15% by weight of thetotal composition. The invention also provides a semiconductor devicemanufactured from the same method.

The invention may also be characterized by the use of a silicon nitridefilm or silicon oxide film as the insulating film. The silicon nitridefilm is typically formed by a plasma chemical vapor deposition (CVD) orthermal CVD process. The silicon oxide film is typically formed bythermal oxidation, thermal CFD or plasma CFD.

The method of manufacture of the semiconductor device may also becharacterized by manufacturing a semiconductor device from a structuralelement composed of a junction-bearing semiconductor substrate and aninsulating film formed on one main surface thereof, wherein theinsulating layer is selected from a titanium oxide silicon nitride,SiNx:H, silicon oxide, and silicon oxide/titanium oxide film, whichmethod includes the steps of forming on the insulating film, in apredetermined shape and at a predetermined position, a metal pastematerial having the ability to react and penetrate the insulating film,forming electrical contact with the silicon substrate. The titaniumoxide film is typically formed by coating a titanium-containing organicliquid material onto the semiconductor substrate and firing, or by athermal CVD. The silicon nitride film is typically formed by PECVD(plasma enhanced chemical vapor deposition). The invention also providesa semiconductor device manufactured from this same method.

The electrode formed from the conductive thick film composition(s) ofthe present invention is typically fired in an atmosphere that ispreferably composed of a mixed gas of oxygen and nitrogen. This firingprocess removes the organic medium and sinters the glass frit with theAg powder in the conductive thick film composition. The semiconductorsubstrate is typically single-crystal or multicrystalline silicon.

FIG. 1A shows a step in which a substrate of single-crystal silicon orof multicrystalline silicon is provided typically, with a texturedsurface which reduces light reflection. In the case of solar cells,substrates are often used as sliced from ingots which have been formedfrom pulling or casting processes. Substrate surface damage caused bytools such as a wire saw used for slicing and contamination from thewafer slicing step are typically removed by etching away about 10 to 20μm of the substrate surface using an aqueous alkali solution such asaqueous potassium hydroxide or aqueous sodium hydroxide, or using amixture of hydrofluoric acid and nitric acid. In addition, a step inwhich the substrate is washed with a mixture of hydrochloric acid andhydrogen peroxide may be added to remove heavy metals such as ironadhering to the substrate surface. An antireflective textured surface issometimes formed thereafter using, for example, an aqueous alkalisolution such as aqueous potassium hydroxide or aqueous sodiumhydroxide. This gives the substrate, 10.

Next, referring to FIG. 1B, when the substrate used is a p-typesubstrate, an n-type layer is formed to create a p-n junction. Themethod used to form such an n-type layer may be phosphorus (P) diffusionusing phosphorus oxychloride (POCl₃). The depth of the diffusion layerin this case can be varied by controlling the diffusion temperature andtime, and is generally formed within a thickness range of about 0.3 to0.5 μm. The n-type layer formed in this way is represented in thediagram by reference numeral 20. Next, p-n separation on the front andbacksides may be carried out by the method described in the backgroundof the invention. These steps are not always necessary when aphosphorus-containing liquid coating material such as phosphosilicateglass (PSG) is applied onto only one surface of the substrate by aprocess, such as spin coating, and diffusion is effected by annealingunder suitable conditions. Of course, where there is a risk of an n-typelayer forming on the backside of the substrate as well, the degree ofcompleteness can be increased by employing the steps detailed in thebackground of the invention.

Next, in FIG. 1D, a silicon nitride film or other insulating filmsincluding SiNx:H (i.e., the insulating film comprises hydrogen forpassivation during subsequent firing processing) film, titanium oxidefilm, and silicon oxide film, 30, which functions as an antireflectioncoating is formed on the above-described n-type diffusion layer, 20.This silicon nitride film, 30, lowers the surface reflectance of thesolar cell to incident light, making it possible to greatly increase theelectrical current generated. The thickness of the silicon nitride film,30, depends on its refractive index, although a thickness of about 700to 900 Å is suitable for a refractive index of about 1.9 to 2.0. Thissilicon nitride film may be formed by a process such as low-pressureCVD, plasma CVD, or thermal CVD. When thermal CVD is used, the startingmaterials are often dichlorosilane (SiCl₂H₂) and ammonia (NH₃) gas, andfilm formation is carried out at a temperature of at least 700° C. Whenthermal CVD is used, pyrolysis of the starting gases at the hightemperature results in the presence of substantially no hydrogen in thesilicon nitride film, giving a compositional ratio between the siliconand the nitrogen of Si₃N₄ which is substantially stoichiometric. Therefractive index falls within a range of substantially 1.96 to 1.98.Hence, this type of silicon nitride film is a very dense film whosecharacteristics, such as thickness and refractive index, remainunchanged even when subjected to heat treatment in a later step. Thestarting gas used when film formation is carried out by plasma CVD isgenerally a gas mixture of SiH₄ and NH₃. The starting gas is decomposedby the plasma, and film formation is carried out at a temperature of 300to 550° C. Because film formation by such a plasma CVD process iscarried out at a lower temperature than thermal CVD, the hydrogen in thestarting gas is present as well in the resulting silicon nitride film.Also, because gas decomposition is effected by a plasma, anotherdistinctive feature of this process is the ability to greatly vary thecompositional ratio between the silicon and nitrogen. Specifically, byvarying such conditions as the flow rate ratio of the starting gases andthe pressure and temperature during film formation, silicon nitridefilms can be formed at varying compositional ratios between silicon,nitrogen and hydrogen, and within a refractive index range of 1.8 to2.5. When a film having such properties is heat-treated in a subsequentstep, the refractive index may change before and after film formationdue to such effects as hydrogen elimination in the electrode firingstep. In such cases, the silicon nitride film required in a solar cellcan be obtained by selecting the film-forming conditions after firsttaking into account the changes in film qualities that will occur as aresult of heat treatment in the subsequent step.

In FIG. 1D, a titanium oxide film may be formed on the n-type diffusionlayer, 20, instead of the silicon nitride film, 30, functioning as anantireflection coating. The titanium oxide film is formed by coating atitanium-containing organic liquid material onto the n-type diffusionlayer, 20, and firing, or by thermal CVD. It is also possible, in FIG.1D, to form a silicon oxide film on the n-type diffusion layer, 20,instead of the silicon nitride film 30 functioning as an antireflectionlayer. The silicon oxide film is formed by thermal oxidation, thermalCVD or plasma CVD.

Next, electrodes are formed by steps similar to those shown in FIG. 1Eand FIG. 1F. That is, as shown in FIG. 1E, aluminum paste, 60, and backside silver paste, 70, are screen printed onto the back side of thesubstrate, 10, as shown in FIG. 1E and successively dried. In addition,a front electrode-forming silver paste is screen printed onto thesilicon nitride film, 30, in the same way as on the back side of thesubstrate, 10, following which drying and firing are carried out in aninfrared furnace at typically at a set point temperature range of 700 to975° C. for a period of from one minute to more than ten minutes whilepassing through the furnace a mixed gas stream of oxygen and nitrogen.

As shown in FIG. 1F, during firing, aluminum diffuses as an impurityfrom the aluminum paste into the silicon substrate, 10, on the backside, thereby forming a p+ layer, 40, containing a high aluminum dopantconcentration. Firing converts the dried aluminum paste, 60, to analuminum back electrode, 61. The backside silver paste, 70, is fired atthe same time, becoming a silver back electrode, 71. During firing, theboundary between the backside aluminum and the backside silver assumesthe state of an alloy, thereby achieving electrical connection. Mostareas of the back electrode are occupied by the aluminum electrode,partly on account of the need to form a p+ layer, 40. At the same time,because soldering to an aluminum electrode is impossible, the silver orsilver/aluminum back electrode is formed on limited areas of thebackside as an electrode for interconnecting solar cells by means ofcopper ribbon or the like.

On the front side, the front electrode silver paste, 500, of theinvention is composed of silver, ZnO, glass frit, organic medium andoptional metal oxides, and is capable of reacting and penetratingthrough the silicon nitride film, 30, during firing to achieveelectrical contact with the n-type layer, 20 (fire through). Thisfired-through state, i.e., the extent to which the front electrodesilver paste melts and passes through the silicon nitride film, 30,depends on the quality and thickness of the silicon nitride film, 30,the composition of the front electrode silver paste, and on the firingconditions. The conversion efficiency and moisture resistancereliability of the solar cell clearly depend, to a large degree, on thisfired-through state.

EXAMPLES

The thick film composition(s) of the present invention are describedherein below.

(I) Survey on Component Contained in Glass Frit

(A) Paste Preparation

Used material in the paste preparation and the content of each componentare as follows.

-   -   i) Electrically Functional Silver Powders: spherical silver        particle [d₅₀ 2.5 μm (as determined with a laser scattering-type        particle size distribution measuring apparatus). The content of        Ag powder was 81.0 parts by weight.    -   ii) Zinc oxide (ZnO) powder: irregular shaped. The content of        ZnO powder was 4.4 parts by weight.    -   iii) Glass Frit: various glass frit as shown in table 1-5 were        prepared to survey the effect of each oxide contained in the        glass frit. The content of glass frit was 2.4 parts by weight.    -   iv) Organic Medium: Ethyl cellulose resin (Aqualon, Hercules)        and Terpineol. The content of organic medium was 9.6 parts by        weight.

Paste preparations were, in general, accomplished with the followingprocedure: The appropriate amount of solvent, medium and surfactant wasweighed then mixed in a mixing can for 15 minutes, then glass frits andmetal additives were added and mixed for another 15 minutes. Since Ag isthe major part of the solids of the present invention, it was addedincrementally to ensure better wetting. When well mixed, the paste wasrepeatedly passed through a 3-roll mill for at progressively increasingpressures from 0 to 400 psi. The gap of the rolls was adjusted to 1 mil.The degree of dispersion was measured by fineness of grind (FOG). Atypical FOG value is generally equal to or less than 20/10 forconductors.

(B) Test Procedure-Contact Resistance (Rc)

Contact Resistance (Rc) was calculated by TLM method. The shape ofelectrode for TLM measurement was 10 mm width and 1 mm length. Thedistance between electrode was 1 mm, 2 mm, 3 mm, 4 mm, and 5 mm.

Firing Conditions: The wafers were fired under the following conditionsusing an IR heating belt furnace.

Maximum set temperature: 870° C.

In-Out time: 120 sec

(C-1) Effect of Bi₂O₃

In order to look into the effect of Bi₂O₃, glass frits with a variety ofthe content of Bi₂O₃ were used to prepare a conductive paste. The Rc offormed electrode was measured in accordance with the aforementionedprocedure. The data is shown at Table 1. The graph ploted with theresult is shown in FIG. 2. As shown in FIG. 2, Rc performance wassuperior in case the content of Bi₂O₃ is more than 5 mol %.

TABLE 1 Bi₂O₃ vs Rc Bi₂O₃ Rc 0.0 300 0.0 200 0.0 76 0.0 300 0.0 78 0.060 0.0 140 0.0 150 16.8 66 8.0 20.0 30.2 45 49.6 19 0.0 45 22.9 46 13.514 17.7 43 18.3 43 34.0 24 9.8 69 19.9 8 12.2 6

(C-2) Effect of BaO

In order to look into the effect of BaO, glass frits with a variety ofthe content of BaO were used to prepare a conductive paste. The Rc offormed electrode was measured in accordance with the aforementionedprocedure. The data is shown at Table 2. The graph ploted with theresult is shown in FIG. 3. As shown in FIG. 3, Rc performance wassuperior in case the content of BaO is less than 5 mol %.

TABLE 2 BaO vs Rc BaO Rc 18.1 300 17.6 200 25.5 76 12.5 300 8.9 78 9.060 10.3 140 18.0 150 0.0 66 0.0 20.0 0.0 45 0.0 19 0.0 45 0.0 46 0.0 140.0 43 0.0 43 0.0 24 0.0 69 0.0 8 0.0 6

(C-3) Effect of SrO

In order to look into the effect of SrO, glass frits with a variety ofthe content of SrO were used to prepare a conductive paste. The Rc offormed electrode was measured in accordance with the aforementionedprocedure. The data is shown at Table 3. The graph ploted with theresult is shown in FIG. 4. As shown in FIG. 4, Rc performance wassuperior in case the content of SrO is less than 5 mol %.

TABLE 3 SrO vs Rc SrO Rc 0.0 300 0.0 200 9.9 76 23.8 300 0.0 78 0.0 600.0 140 0.0 150 0.0 66 0.0 20.0 0.0 45 0.0 19 0.0 45 0.0 46 0.0 14 0.043 0.0 43 0.0 24 0.0 69 0.0 8 0.0 6

(C-4) Effect of B₂O₃

In order to look into the effect of B₂O₃, glass frits with a variety ofthe content of B₂O₃ were used to prepare a conductive paste. The Rc offormed electrode was measured in accordance with the aforementionedprocedure. The data is shown at Table 4. The graph ploted with theresult is shown in FIG. 5. As shown in FIG. 5, Rc performance wassuperior in case the content of B₂O₃ is less than 15 mol %.

TABLE 4 B₂O₃ vs Rc B₂O₃ Rc 42.9 66 8.8 20 39.0 45 29.7 19 17.1 45 28.646 16.2 14 28.1 43 23.2 43 30.6 24 29.8 69 7.9 8 5.3 6

(C-5) Effect of Al₂O₃

In order to look into the effect of Al₂O₃, glass frits with a variety ofthe content of Al₂O₃ were used to prepare a conductive paste. The Rc offormed electrode was measured in accordance with the aforementionedprocedure. The data is shown at Table 5. The graph ploted with theresult is shown in FIG. 6. As shown in FIG. 6, Rc performance wassuperior in case the content of Al₂O₃ is less than 5 mol %.

TABLE 5 Al₂O₃ vs Rc Al₂O₃ Rc 7.9 66 2.8 20 0.0 45 0.0 19 2.1 45 18.4 460.0 14 11.2 43 18.3 43 7.6 24 10.3 69 0.4 8 0.8 6(II) Survey on Zn/Ag

(A) Paste Preparation

Used material in the paste preparation and the content of each componentare as follows.

-   -   i) Electrically Functional Silver Powders: spherical silver        particle [d₅₀ 2.5 μm (as determined with a laser scattering-type        particle size distribution measuring apparatus). The content of        Ag powder was 84.0 parts by weight.    -   ii) Zinc oxide (ZnO) powder irregular shaped. The content of ZnO        was 0.0 to 5.5 parts by weight.    -   iii) Glass Frit: various glass frit as shown in table 1-5 were        prepared to survey the effect of each oxide contained in the        glass frit. The content of glass frit was 2.5 parts by weight.    -   iv) Organic Medium: Ethyl cellulose resin (Aqualon, Hercules)        and Terpineol. The content of organic medium was 20.2 to 23.2        parts by weight.

The content of ZnO was adjusted so as to fall in the range of 0.0 to 7.7in terms of (ZnO/Ag)×100.

(B) Test Procedure

Solar cells were prepared in accordance with the following procedure.

First, the Si substrate was prepared. On the backside of this Sisubstrate, the electrically conducting paste (the silver paste) for thesolder connection use was coated by screen printing and dried. Then,Aluminum paste PV333 (commercially available from E. I. du Pont deNemours and Company) for the back electrode was coated by screenprinting and dried so that it partly overlapped with the dried silverpaste. The drying temperature for each paste was 120° C.

Furthermore, the coating was carried out so that after drying the filmthickness of each electrode on the back, was 35 μm for the aluminumpaste and 20 μm for the silver paste.

Furthermore, the paste of the present invention was coated on thelight-receiving surface by screen printing and dried by use of aprinting machine manufactured by Price Company. A stainless wire 250mesh with a 8″×10″ frame was used as the mesh. The test pattern was a1.5 inch square consisting of TLM pattern which had electrode as 10 mmwidth and 1 mm length. The distance between electrode was 1 mm, 2 mm, 3mm, 4 mm and 5 mm. The film thickness after firing was 13 μm.

The resulting substrate was subjected to simultaneous firing of thecoated pastes in an infrared furnace with a peak temperature of about870° C. and IN-OUT for about 2 minutes to obtain the desired solar cell.

The resistance between electrode were measured using a model R6871Emultimater manufactured by ADVANTEST Co.

(C-6) Influence of ZnO/Ag

In order to look into the relationship between electric characteristicsand ZnO/Ag, pastes with a variety of the value of ZnO/Ag was prepared.The conversion efficiency (%) of formed electrode was measured inaccordance with the aforementioned procedure. Five samples were preparedfor each paste, and the average value for the five samples was used. Theresults are shown in FIG. 7. Efficiency is shown as a relative value (r%). As shown in FIG. 7, Rc performance was superior in case the contentof (ZnO/Ag)×100 was more than 2.5.

1. A thick film conductive composition comprising: a) electricallyconductive silver powder; b) ZnO powder; c) lead-free glass fritswherein based on total glass frits: Bi₂O₃>5 mol % B₂O₃<15 mol % BaO<5mol % SrO<5 mol % Al₂O₃<5 mol %; and d) organic medium, wherein (thecontent of ZnO/the content of the silver powder)×100 is more than 2.5.2. The composition of claim 1 further comprising an additionalmetal/metal oxide additive selected from the group consisting of (a) ametal selected from Zn, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, and Cr;(b) an oxide of a metal selected from Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co,Fe, Cu and Cr; (c) any compounds that can generate the metal oxides of(b) upon firing; and (d) mixtures thereof.
 3. The thick film compositionof claim 1 wherein the lead-free glass frit has the followingcomposition based upon total glass frits Bi₂O₃ 10.0-60 mol % B₂O₃ isless than 10 mol % BaO is less than 2 mol % SrO is less than 2 mol %Al₂O₃ is less than 2 mol %.
 4. The thick film composition of claim 1wherein the average glass particle size is in the range of 1.0-4.0 μm.5. The thick film composition of claim 1 wherein the glass frit in thecomposition is 0.5 to 7.0 wt % of the total composition.
 6. A substratewherein the composition of claim 1 has been deposited thereon andwherein said composition has been processed to remove said organicmedium and sinter said glass frit and silver powder.
 7. An electrodeformed from the composition of claim 1 wherein said composition has beenfired to remove the organic vehicle and sinter said glass particles. 8.A semiconductor device from the composition of claim
 1. 9. Asemiconductor device formed from the composition of claim 1 wherein saidcomposition has been processed to remove said organic medium and sintersaid glass frit and silver powder.
 10. A method of manufacturing asemiconductor device from a structural element composed of asemiconductor having a p-n junction and an insulating film formed on amain surface of the semiconductor comprising the steps of: (a) applyingonto said insulating film the thick film composition of claim 1; and (b)firing said semiconductor, insulating film and thick film composition toform an electrode.
 11. The method of claim 10, wherein the insulatingfilm is selected from the group consisting of silicon nitride film,titanium oxide film, SiNx:H film, silicon oxide film and a siliconoxide/titanium oxide film.
 12. A substrate coated with a compositioncomprising: a) electrically conductive silver powder; b) ZnO powder; c)lead-free glass frits wherein, based on total glass frits: Bi₂O₃ is 10.0to 60 mol % B₂O₃<10 mol % BaO<2 mol % SrO<2 mol % Al₂O₃<2 mol %; and d)organic medium, wherein the content of ZnO divided by the content of thesilver powder×100 is more than 2.5 and wherein the composition has beenprocessed to remove the organic medium and sinter the glass frit andsilver powder.
 13. A thick film conductive composition consisting of: a)electrically conductive silver powder; b) ZnO powder; c) lead-free glassfrits wherein based on total glass frits: Bi₂O₃>5 mol % B₂O₃<15 mol %BaO<5 mol % SrO<5 mol % Al₂O₃<5 mol %; and d) organic medium, wherein(the content of ZnO/the content of the silver powder)×100 is more than2.5 and further consisting of an additional metal/metal oxide additiveselected from the group consisting of (a) a metal selected from Zn, Gd,Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, and Cr; (b) an oxide of a metalselected from Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu and Cr; (c) anycompounds that can generate the metal oxides of (b) upon firing; and (d)mixtures thereof, wherein the particle size of the additional metaladditive is no more than 10 μm.